1. Field of the Invention
The present invention relates to a method and apparatus for determining average borehole diameter or caliber during a drilling operation, and in particular to a method which can be carried out utilizing any known borehole compensated downhole measurement device.
2. Description of the Prior Art
In any well drilling operation, it is necessary to constantly monitor the condition of the borehole in order to provide early detection of conditions which may require extra steps in order to stabilize the walls of the borehole. For example, a particular formation may have a tendency to swell, which could cause a narrowing of the borehole and possibly the entrapment of the downhole assembly or fracture of the formation face due to excessive bottom pressures. Another example would be a cavity in a formation and which would generate additional debris to be removed from the borehole. Corrective steps which can be taken include modifying the properties of the drilling mud, withdrawing the drill string to rebore a narrowing formation and/or inserting a well casing and filling the annulus between it and the borehole wall with cement to stabilize the borehole. In a cementing operation it is also important to know the diameter of the annulus to be filled so as to determine the volume of cement which will be required and when the cementing operation is completed.
Heretofore, most of the borehole calibration devices have been associated with wireline well logging devices. While many of these provide very accurate borehole calibration, the information is not generated until after the drilling operation has been interrupted, the drill string removed from the borehole and the wireline device lowered downhole. This is a time consuming and expensive operation and points out the need for a method and apparatus for determining the borehole caliber while the drilling operation continues so as to provide the operator with real time information and enabling corrective action to be taken promptly. Many of the above-mentioned wireline devices encounter problems with mud cake, which builds up during the drilling operation, since they require physical contact with the borehole wall, as for example with a six arm caliber or asymmetrically operated devices which actually penetrate the mud cake.
The borehole caliber measurement is utilized in interpreting some well logs and as a correction factor in other well logs, such as nuclear logs, acoustic logs and diameters. Thus, a correct and current measurement of borehole caliber is very important in properly evaluating the potential productivity of the well.
Since there is only a limited amount of room available in a measuring-while-drilling downhole tool, it is important to obtain the maximum amount of information possible with the most efficient utilization of the downhole equipment. The present invention accomplishes this by employing existing downhole measurement devices in a novel manner to make borehole calibration measurements while drilling.